1. Field of the Invention
This invention relates generally to nuclear reactors, and more particularly to a rod assembly for a nuclear reactor.
2. Description of the Related Art
A continuing problem during operation of a nuclear reactor is the existence of debris of various sizes. Examples of such debris may include small-sized fasteners, metal clips, welding slag, pieces of wire, etc. The debris may be generated as a result of the original construction of the reactor core, subsequent reactor operation and/or due to repairs made during a planned or unplanned maintenance outage.
During the operation of such nuclear reactors, the debris may be carried by the cooling water (reactor coolant) and may become wedged between or within reactor components such as fuel rods of a fuel assembly in a boiling water reactor (BWR) or pressurized water reactor (PWR), or in control rod assemblies in a PWR, etc. The repeated interaction between the entrained debris and component(s) can result in fretting damage to the component(s), such as damage to the fuel rods.
Some of the debris may be caught between the fuel rods and other fuel assembly components. The debris vibrates in the moving coolant and interacts with the fuel rods, potentially causing what is known as fretting wear of the fuel rod cladding. This fretting may be recognized as a significant cause of failure of the fuel rod in a BWR or PWR, for example.
Conventional solutions have included employing the use of debris filters to filter the debris from the reactor coolant. These are typically positioned in the lower tie plate or nozzle of a fuel assembly and so are replaced when the fuel is discharged. Debris filtering devices have also been introduced into the nuclear plant piping system. However, the debris filter and/or external filter mechanisms do not completely obviate the fretting problem in nuclear reactors. Fretting still may occur and cause fuel failures, which may release fuel, fission products or other rod contents into the coolant, potentially leading to the premature withdrawal from service of the fuel assembly or costly mid-cycle fuel replacement.